Optical-resolution photoacoustic microscopy offers a specific contrast to optical absorption. The limiting penetration depth of current techniques due to scattering produced by tissues makes endoscopic approaches attractive for photoacoustic imaging deep inside biological structures. Conventional approaches generally involves mechanically raster scanning a focused spot over the sample and acquiring an acoustic signal for each spot. Here, we demonstrate that a full-field illumination approach with multiple known speckle patterns generated by a multimode fiber can also provide diffraction-limited optical-resolution photoacoustic images. As a proof of principle we experimentally image micro-structured test samples illuminated with reference speckle patterns measured during a calibration step. A digital micromirror device modulating the incident light coupled into a multimode fiber provides the different speckle patterns at the distal tip of the fiber where the sample is placed.
We study and compare the performance in simulations and experiments of three different approaches; the first method is based on cross-correlation between the photoacoustic signal under multiple speckle illumination with the calibrated known speckle patterns, following approaches from ghost imaging. The second method is based on computing the pseudo-inverse of the reference matrix obtained from the calibration step. A third method based on compressed sensing exploits the sparsity of the sample achieving reconstructed images with a number of speckle realizations smaller than the number of speckle grains. Additionally, speckle-illumination-based photoacoustic microscopy provides a powerful framework for the development of novel reconstruction approaches, that can demand less computation time in case of compressed sensing approaches.
Antonio M. Caravaca-Aguirre, Florian Poisson, and Emmanuel Bossy, "Speckle based optical-resolution photoacoustic endoscopy (Conference Presentation)," Proc. SPIE 10494, Photons Plus Ultrasound: Imaging and Sensing 2018, 1049415 (Presented at SPIE BiOS: January 29, 2018; Published: 15 March 2018); https://doi.org/10.1117/12.2287767.5752212563001.
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